An OLED device includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron-transporting layer, and a cathode. OLED devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. Tang et al. described this multilayer OLED device in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
Physical vapor deposition in a vacuum environment is the principal means of depositing thin organic material films as used in small molecule OLED devices. Such methods are well known, for example Barr in U.S. Pat. No. 2,447,789 and Tanabe et al. in EP 0 982 411. The organic materials used in the manufacture of OLED devices are often subject to degradation when maintained at or near the desired rate dependant vaporization temperature for extended periods of time. Exposure of sensitive organic materials to higher temperatures can cause changes in the structure of the molecules and associated changes in material properties.
To overcome the thermal sensitivity of these materials, only small quantities of organic materials have been loaded in sources and they are heated as little as possible. In this manner, the material is consumed before it has reached the temperature exposure threshold to cause significant degradation. The limitations with this practice are that the available vaporization rate is very low due to the limitation on heater temperature, and the operation time of the source is very short due to the small quantity of material present in the source. In the prior art, it has been necessary to vent the deposition chamber, disassemble and clean the vapor source, refill the source, reestablish vacuum in the deposition chamber and degas the just-introduced organic material over several hours before resuming operation. The low deposition rate and the frequent and time consuming process associated with recharging a source has placed substantial limitations on the throughput of OLED manufacturing facilities.
A secondary consequence of heating the entire organic material charge to roughly the same temperature is that it is impractical to mix additional organic materials, such as dopants, with a host material unless the vaporization behavior and vapor pressure of the dopant is very close to that of the host material. This is generally not the case and, as a result, prior art devices frequently require the use of separate sources to co-deposit host and dopant materials.
A consequence of using single component sources is that many sources are required in order to produce films containing a host and multiple dopants. These sources are arrayed one next to the other with the outer sources angled toward the center to approximate a co-deposition condition. In practice, the number of linear sources used to co-deposit different materials has been limited to three. This restriction has imposed a substantial limitation on the architecture of OLED devices, increases the necessary size and cost of the vacuum deposition chamber and decreases the reliability of the system.
FIG. 1 shows a cross-sectional view of a conventional vaporization device 5, which includes three individual sources 6, 7, and 8, commonly termed “heating boats”, for vaporizing organic material. Vapor plume 9 is preferably homogeneous in the materials from the different sources, but in fact varies in composition from side to side resulting in a non-homogeneous coating on substrate 15.
Among the acknowledged problems of conventional approaches to vapor deposition are material purity and the difficulty in maintaining a continuous fabrication sequence that would allow deposition apparatus to operate for extended periods of time, without costly down-time for cleaning, for replenishment or recharging of material, and for pre-heating. Both of these problems present significant constraints on deposition system design. Conventional solutions reflect the assumption that there would be no suitable solution that would allow both continuous (or, at least, longer term) deposition processing and maintenance of a high level of material purity.
Conventionally, for example, the problem of maintaining high material purity is solved somewhat indirectly. As source 6 (FIG. 1), a crucible or “boat” is initially charged with material in an unheated state. Then, before deposition can begin, a “bake-out” of the material occurs in a pre-treatment or pre-heating cycle as the deposition chamber heats up. During this bake-out period, moisture and volatile impurities are thus effectively drawn off from the material. Then, when vaporization device 5 is up to operating temperature, the material in source 6 is relatively pure. With this type of solution, the material to be deposited requires careful handling, but generally need not be hermetically sealed beforehand against contamination, such as in shipping or during preparation, for example.
With the intention of reducing this bake-out time period and improving overall deposition efficiency, there have been a number of proposed solutions for providing material in a more purified form. For example, U.S. Published Patent Application No. 2004/0255857 entitled “Thin-Film Deposition Evaporator” by Chow et al. discloses a moving vessel that acts as a type of “shuttle” for replenishing the supply of vaporizable material in a deposition chamber. The method of the '5857 Chow et al. solution addresses the problems of maintaining a suitable vacuum level in the deposition system during replenishment and reducing the overall likelihood of contamination of the material. However, the solution proposed in the '5857 Chow et al. disclosure is not advantaged for any type of continuous processing because this batch replenishment process still requires a pre-heating period during which material in the deposition chamber heats up to deposition temperature.
Another proposed solution for improving the purity of materials used in replenishment is given in U.S. Patent Published Application No. 2004/0139914 by Yamazaki et al. As one part of the '9914 Yamazaki et al. solution, it is proposed to provide a supply crucible or other source that already contains replenishment material, hermetically sealed to prevent contamination, for placement directly into the crucible for deposition. While this type of solution can help to improve overall material purity, however, it still requires that the deposition process be stopped in order to allow replenishment, then restarted again once the sealed source is placed into position, the seal broken, and the material heated to deposition temperatures.
Thus, it can be seen that while solutions such as those proposed in the '5857 Chow et al. and '9914 Yamazaki et al. disclosures may serve to improve material purity to some degree and even minimize bake-out pre-treatment requirements, these solutions do not wholly eliminate the pre-heating cycle. Moreover, neither of these solutions would be suitable for use with a materials replenishment system that supports continuous operation, so that processing can continue while materials recharging takes place.
A shortcoming that is inherent to any method that requires a pre-treatment or pre-heating cycle relates to the need for simultaneous deposition of multiple component materials. In some applications, it is advantageous to deposit more than one material at a time. It may even be beneficial to mix two or more materials in the same container for vaporization. However, because some amount of pre-heating or bake-out time is required with conventional techniques, both materials must be substantially of the same sublimation temperature (to within a small tolerance in the range of no more than about ±20 degrees C. difference). Where materials differ in vaporization temperature by more than a few degrees, conditions for pre-heating or bake-out may not be optimal.
Thus, it can be seen that there is a need for a replenishment solution in a vapor deposition system that allows recharging of material without interrupting the deposition operation and that allows material to be provided in a highly purified form so that pre-treatment, bake-out, or pre-heating requirements are effectively eliminated.